Eye consists mainly of Eyeball, Ocular adnexa (eyelids, lacrimal apparatus and orbit), their vascular connections and neurological pathways.
Structure (Fig. 1)
Structurally the eyeball has three walls performing different visually vital functions, enclosing various intraocular structures like lens, vitreous cavity enclosing the vitreous and the anterior and posterior chambers filled with aqueous, arranged in order so that light comes to a focus on the retina resulting in image formation. The walls of the eye are as follows:
- Outer wall: Is mainly protective in function and is made of cornea in front and sclera behind.
- Middle wall: Gives nutrition to many parts of eye and has three portions— iris, ciliary body and choroid.
- Inner wall: Performs predominantly neurological function—the retina.
Eye ball is not a perfectly spherical globe, as its anterior and posterior segments have different curvatures.
Anterior 1/6th of outer wall, the cornea is more curved (steeper) than posterior 5/6th, the sclera which is less curved (flatter) with a transitional zone in between them (limbus)(Fig. 2).
Radius of Curvature of Globe
- Radius of curvature of the anterior portion (cornea) is about 7.8 mm.
- Radius of curvature of the posterior portion (sclera) is about 12 mm.Summit of the curved anterior portion is the anterior pole and the apex of curved posterior portion is the posterior pole of the globe.
Diameter of Eyeball
Anteroposterior diameter (length) of eye ball is about 24 mm. Vertical diameter is 23.5 mm.
Equator of the Eyeball
- External (Geometric) equator: Externally the equator of the eye ball is a circular line drawn on the exterior surface of the ball equidistant from anterior and posterior pole. The circumference of equator is about 71 to 75 mm.
- Anatomical equator: Since the eye is slightly anatomically tilted backwards on temporal side for 1 to 1.5 mm and slightly nasally and sclera buldges slightly temporally, circle representing the anatomical equator may not be exactly in the middle of the globe.
- Surgical equator: Is the widest diameter of the globe in coronal plane.
- Clinical equator: Is an imaginary line drawn along the ampullae of vortex vein as seen on fundus examination (Fig. 3).
Center of (geometrical) eyeball: Center of eye ball is about 12 mm behind the anterior pole of globe.
Center of rotation of eyeball: Center of rotation of eyeball is slightly behind the center of the eyeball and is about 13 mm behind the anterior pole.
Nodal point of the eyeball: Different points of visual field are connected to the retinal points by visual planes, each of them passing through the nodal point of eye. This point is situated in the posterior pole of the lens and is about 7.2 mm behind the apex of cornea (Fig. 4).
Visual axis is an imaginary line connecting object of fixation, nodal point of the eye and fovea.
Optic axis passes from a point 0.25 mm nasal to the center of cornea, lens, center of rotation of eyeball and approximately through the center of pupil intersecting visual axis at nodal point and extent more nasally posteriorly. Pupillary line passes through center of cornea and center of pupil almost near the optic axis.
The angle between visual axis and optic axis is called angle gamma. This angle is assessed clinically at the pupillary plane (not very accurate) and is called angle Kappa also (Figs. 5 and 6).
SUMMARY OF GENERAL MEASUREMENTS OF EYEBALL (FIG. 7)
12 mm behind apex of cornea
7.2 mm behind apex of cornea
13 mm behind anterior pole
7.15 grams in adult and 3 grams in infant